Sadly for large swaths of Southern California, it would be easy to at least keep people from shooting by enforcing at a limited number of areas. Off the top of my head there's the Angeles range, Lytle creek and Barstow area, and the place in Corona(rauhags?) where rifles can be shot legally and are easily accessible(Applevalley too if you include membership clubs). Crack down on those, and people have to drive into NV/AZ or have access to a vehicle capable of really getting out there in the high desert.